internal/coverage/slicewriter: fix off-by-1 error in seek utilities

The slicewriter Seek method was being too restrictive on offsets
accepted, due to an off-by-one problem in the error checking code.
This fixes the problem and touches up the unit tests.

Change-Id: I75d6121551de19ec9275f0e331810db231db6ea9
Reviewed-on: https://go-review.googlesource.com/c/go/+/488116
Run-TryBot: Than McIntosh <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: thanm

src/internal/coverage/slicewriter/slicewriter.go

@@ -38,21 +38,21 @@ func (sws *WriteSeeker) Write(p []byte) (n int, err error) {
 func (sws *WriteSeeker) Seek(offset int64, whence int) (int64, error) {
 	switch whence {
 	case io.SeekStart:
-		if sws.off != offset && (offset < 0 || offset >= int64(len(sws.payload))) {
+		if sws.off != offset && (offset < 0 || offset > int64(len(sws.payload))) {
 			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(sws.payload))
 		}
 		sws.off = offset
 		return offset, nil
 	case io.SeekCurrent:
 		newoff := sws.off + offset
-		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+		if newoff != sws.off && (newoff < 0 || newoff > int64(len(sws.payload))) {
 			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
 		}
 		sws.off += offset
 		return sws.off, nil
 	case io.SeekEnd:
 		newoff := int64(len(sws.payload)) + offset
-		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+		if newoff != sws.off && (newoff < 0 || newoff > int64(len(sws.payload))) {
 			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
 		}
 		sws.off = newoff

src/internal/coverage/slicewriter/slw_test.go

@@ -47,12 +47,13 @@ func TestSliceWriter(t *testing.T) {
 		sleq(t, b, p)
 	}
 
-	sk := func(t *testing.T, ws *WriteSeeker, offset int64, whence int) {
+	sk := func(t *testing.T, ws *WriteSeeker, offset int64, whence int) int64 {
 		t.Helper()
-		_, err := ws.Seek(offset, whence)
+		off, err := ws.Seek(offset, whence)
 		if err != nil {
 			t.Fatalf("unexpected seek error: %v", err)
 		}
+		return off
 	}
 
 	wp1 := []byte{1, 2}
@@ -80,6 +81,8 @@ func TestSliceWriter(t *testing.T) {
 	rf(t, ws, []byte{2, 7})
 	sk(t, ws, -4, io.SeekEnd)
 	rf(t, ws, []byte{2, 7})
+	off := sk(t, ws, 0, io.SeekEnd)
+	sk(t, ws, off, io.SeekStart)
 
 	// seek back and overwrite
 	sk(t, ws, 1, io.SeekStart)
@@ -98,7 +101,7 @@ func TestSliceWriter(t *testing.T) {
 	if err == nil {
 		t.Fatalf("expected error on invalid -1 seek")
 	}
-	_, err = ws.Seek(int64(len(ws.BytesWritten())), io.SeekStart)
+	_, err = ws.Seek(int64(len(ws.BytesWritten())+1), io.SeekStart)
 	if err == nil {
 		t.Fatalf("expected error on invalid %d seek", len(ws.BytesWritten()))
 	}
@@ -108,7 +111,7 @@ func TestSliceWriter(t *testing.T) {
 	if err == nil {
 		t.Fatalf("expected error on invalid -1 seek")
 	}
-	_, err = ws.Seek(int64(len(ws.BytesWritten())), io.SeekCurrent)
+	_, err = ws.Seek(int64(len(ws.BytesWritten())+1), io.SeekCurrent)
 	if err == nil {
 		t.Fatalf("expected error on invalid %d seek", len(ws.BytesWritten()))
 	}